Asphalt

Asphalt, ASPHALTUM, or MINERAL PITCH, is the name given to a compact form of bitumen, which is usually black or dark-brown in colour. When free from earthy impurities, it has a conchoidal fracture and resinous lustre. Asphalt is generally found wherever rock-oil occurs, and in such localities it is clearly produced by the drying up of the petroleum. In some places, however, it occurs in beds forming a compact rock. The Dead Sea, the district near Babylon, some of the West Indian Islands, notably the Pitch Lake in Trinidad, and one or two places in France, Switzerland, and Dalmatia, are the best-known localities for this substance; but it is found, more or less, in a great many countries. Asphalt was employed by the ancient Egyptians for embalming their dead, and it was used in Babylon as mortar. Its modern applications are numerous. It is an ingredient in Japan varnish, and is used along with other materials to make waterproof roofing and flooring, linings for cisterns, and along with pasteboard material in the construction of water-pipes. It is much used to form what are called 'damp courses' in walls of buildings—that is, a layer of it, from $\backslash frac\{1\}\{2\}$ inch to $\backslash frac\{3\}\{4\}$ inch thick, is spread over the thickness of a wall near the ground-level, to prevent the ascent of damp. Frequently nowadays the whole internal area of a house is covered with a layer of asphalt. In cases where the wall of a house comes against a bank of earth, the whole surface is protected from damp by a lining of this material. One or two kinds, such as those found at Seyssel in the east of France, and at Val-de-Travers in Switzerland, though called asphalts, are really bituminous limestones. The latter is known all over the world as a material for pavement. This Val-de-Travers asphalt is prepared by reducing the natural rock, which contains from 7 to 29 per cent. of bitumen, to powder, and then putting it with a small quantity of melted bitumen into a caldron. After it is fused and stirred for some time, it is run into moulds to form blocks of about 1 cwt. each. These blocks are called 'asphaltic mastic,' and the finest kinds contain 87 per cent. of carbonate of lime and 13 of bitumen. This mastic should not melt below 168° F. It has, especially since 1854, been very extensively employed in the construction of pavements. When this material is used, there is, of course, far less noise produced by the traffic on the streets than with stone. For paving purposes, the 'asphaltic mastic' is heated in portable boilers, into which, at a certain stage of the preparation, there is poured about 25 per cent. of thoroughly dried sand, gravel, or powdered limestone, which is well mixed with the liquid asphalt. The mixture is then spread on the spot prepared for it, and when cool forms a hard kind of pavement. In Paris, both for carriage-ways and foot-pavements, it has been largely employed for more than thirty years, having been introduced by Napoleon III. with a view, it is said, to prevent the erection of barricades with paving-stones. In more recent times it has been extensively used in Berlin, and many other continental towns, for the same purposes. In London and other parts of Great Britain, foot-pavements are still frequently made of it, but it has been but very partially used for carriage-ways. For this last purpose the moist climate of our island probably renders it more slippery than on the Continent. Pavements formed of an artificial or coal-tar asphalt have long been, to a limited extent, in use; but this material is not so suitable for them as the natural asphalts (see GAS-TAR). It is well to state, however, that artificial asphalt is more used for 'damp courses' than the asphalts from bituminous limestones, although the latter are much better. Of late years, an asphalt made of coal-tar pitch and a cheap mineral oil called creosote oil, has been much used for the joints of wood-pavement and causeway stones, and does very well. The pigment known as asphaltum is sometimes prepared from natural asphalt, but more frequently from the residue of distilled bituminous substances. Unfortunately, its fine transparent brown colour has tempted some distinguished modern artists to use it largely. Through its property of not drying thoroughly and free of cracks, a number of fine pictures painted some years ago by Horace Vernet, Sir George Harvey, and others, are now mere wrecks. For the synonyms of asphalt, see BITUMEN.